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Travel Epidemiology

Mark J. Sotir, Regina C. LaRocque

Travelers are an epidemiologically important population because of their mobility, the potential for exposure to diseases and other adverse health outcomes outside their home country, and the possibility that they will either import nonendemic diseases into their country of origin or export nonendemic diseases to the country they visit. International tourist arrivals exceeded 1 billion in 2012 and are projected to increase to almost 2 billion by 2030, so the public health impact of travel will only increase. Increasing travel to destinations in Asia (arrivals up 7% from 2011 to 2012) and Africa (arrivals up 6% from 2011 to 2012) will place more travelers at risk for a variety of travel-related conditions, including malaria, dengue, measles, and other tropical or vaccine-preventable infections.

To provide optimal pre-travel advice, preventive measures, and education to travelers, health care providers must be familiar with basic epidemiologic concepts and the epidemiology and geographic distribution of relevant diseases and conditions. Risk can be assessed by examining both past and current disease trends. However, there are limitations to the data available for making such an assessment. Knowledge of the precise risk for a particular disease in a specific location, especially pertinent for a traveler, remains elusive for many destinations despite decades of interest and improvements in global surveillance efforts.

Information regarding the actual risk for travelers (often expressed as number of events per 100,000 travelers) is limited for a number of reasons. It is difficult to obtain an accurate numerator (number of cases of disease among travelers) and denominator (number of travelers overall or travelers to a specific destination who are susceptible to infection and illness). Many travelers to a specific location who become infected have already returned to their home country by the time they develop signs and symptoms, so will not be included in the surveillance data of the country of exposure even if reporting is optimal. Similarly, diseases with short incubation periods or brief durations may have resolved by the time a traveler returns home and thus may not be counted in surveillance data of the traveler’s country of origin. If the illness is mild, the traveler may never seek health care, or diagnostic tests may not be performed to accurately diagnose the cause. Travelers often visit multiple locations, and it may be difficult to determine where the exposure occurred. A denominator for all travelers to a specific location that could be used to calculate incidence is also generally problematic. Even when available, denominator data are typically estimates provided only at the country or region level and not at the local destination level.

Frequently quoted studies on the incidence of infection in travelers are based on extrapolations of limited data, collected in limited samples of travelers. In addition, these studies were often conducted >20 years ago and might be of limited relevance to current travelers. These studies also use a variety of methodologic designs, each with its own set of strengths and weaknesses, making the findings difficult to compare or combine. They have also, for the most part, only examined a few key diseases or conditions and have combined all travelers regardless of destination. Many have been single-clinic or single-destination studies that lead to conclusions that are not applicable to groups of travelers with different local, national, or cultural backgrounds.

A number of factors are relevant to epidemiologic data on travel-related diseases and adverse health events. First, the characteristics of the disease itself must be considered, including mode of transmission, incubation period, signs and symptoms, duration of illness, and diagnostic testing. Second, the presence, frequency, seasonality, and geographic distribution of the disease need to be assessed; these might change over time because of outbreaks, emergence or reemergence in new areas or populations, successful public health interventions, or other factors. Third, travelers represent a unique subset of people, and their exposures, behaviors, and disease susceptibility might differ dramatically from those of the local population at a tourist destination. Along with demographic characteristics, additional travel-specific factors that should be considered include trip length, destinations (both current and previous), specific travel itineraries, and purpose of travel. Fourth, travelers themselves are a heterogeneous group, and different subgroups of travelers might have different risks because of activities, behaviors, and other factors during travel. For example, travelers visiting friends and relatives (VFR travelers) have consistently demonstrated higher proportions of serious febrile illness, particularly malaria, when compared with other types of travelers. Finally, preparation before travel, especially use of preventive measures such as vaccines, also factors into the epidemiology of travel illnesses.

During the past 2 decades, the most relevant data on travel-related disease occurrence have come from surveillance of travelers themselves. Data on disease incidence in local populations may identify the most important diseases to monitor in a country, but their relevance to travelers who have different risk behaviors, eating habits, accommodations, knowledge of preventive measures, and activities is usually limited. Surveillance data that either focus on travelers or collect data on illnesses that affect travelers, and incorporate some of the travel-specific variables described above in their data collection methods, are more useful in describing travel-related disease patterns and risks.

One example of a surveillance data source is the GeoSentinel Surveillance Network, an ongoing collaborative effort between the International Society of Travel Medicine (ISTM) and CDC. GeoSentinel is a global provider-based network of travel and tropical medicine clinics that has been systematically collecting data on travel-related illnesses since 1997. As of 2014, GeoSentinel had 57 participating clinics on 6 continents, with a database of >200,000 records on patients with a confirmed or probable travel-related diagnosis. Details of the data collection instrument, diagnostic categories, and patient classification methods used in GeoSentinel have recently been published in a CDC Morbidity and Mortality Weekly Report (MMWR) surveillance summary. Further information on GeoSentinel can be found at: www.istm.org/geosentinel.

Recent data from the GeoSentinel network indicate that Asia (32.6%) and sub-Saharan Africa (26.7%) were the most common regions where travel-related illnesses were acquired (Figure 1-5). The most frequently identified illness categories were gastrointestinal (34.0%), febrile (23.3%), and dermatologic (19.5%) problems. Malaria, dengue, enteric fever, spotted-fever group rickettsioses, chikungunya, and nonspecific viral syndromes were the most frequent contributors to the acute systemic febrile illness category. Falciparum malaria was most commonly acquired in West Africa, while enteric fever was most often contracted on the Indian subcontinent; leptospirosis, scrub typhus, and murine typhus were principally acquired in Southeast Asia. More than two-thirds of dengue infections were acquired in Asia, mostly Thailand, Indonesia, and India; seasonality of dengue varied according to destination. Common skin and soft tissue infections, mosquito bites (often infected), and allergic dermatitis were the most common skin conditions affecting travelers. Among the more exotic diagnoses, hookworm-related cutaneous larva migrans, leishmaniasis, myiasis, and tungiasis were the most important. The relative frequency of many diseases varied by travel destination and reason for travel, and VFR travelers had a disproportionately high prevalence of serious febrile illness (malaria) and low rates of seeking advice before travel (18.3%). Only 40.5% of all ill GeoSentinel travelers reported pre-travel medical visits.

Although GeoSentinel and similar traveler surveillance networks, such as TropNet in Europe, offer advantages over disease-specific studies or data collated at single centers, these networks have limitations. The data are based on a convenience sample of ill returned travelers visiting specialist clinics. This group may not reflect the experience of healthy travelers, those with mild or self-limited illness who visit other providers, or those who do not seek health care when ill. In addition, there are differences in patient populations and travel destinations among the GeoSentinel participating sites. Because of a lack of a true denominator in GeoSentinel data, they cannot be used to determine absolute or relative risks. However, they show the relative frequency and range of illnesses seen in wide samples of ill travelers.

Another valuable source of epidemiologic data is Global TravEpiNet (GTEN), a US-based provider network formed in 2009 that systematically collects information on travelers presenting to a consortium of 26 clinics, including specialized travel clinics, primary care and public health sites, and pharmacy-based practices. GTEN providers evaluate the health characteristics and pre-travel health care of travelers and document these in a central electronic database; >51,000 records have been collected as of January 1, 2014. GTEN does not capture information on travelers who seek pre-travel care overseas, and participating sites may not be representative of clinical practice in all parts of the United States. More information on Global TravEpiNet, its web tools, and its membership can be found at www.healthful.travel.

Data from 13,235 travelers seen from 2009 through 2011 in GTEN clinics indicated that India, South Africa, and China were the most common destinations for US travelers seeking pre-travel health care, and more than one-third of trips occurred in June, July, and August. Travelers seen in sampled GTEN clinics ranged in age from 1 month to 94 years, with a median of 35 years. The median duration of travel was 14 days, although 22% of travelers pursued trips of >28 days, and 3% of travelers pursued trips of >6 months. A total of 75% were traveling to malaria-endemic countries, and 38% were visiting yellow fever–endemic countries. Immunocompromising conditions, such as HIV infection and AIDS, organ transplant, or receipt of immunocompromising medications, were present in 3% of GTEN travelers.

Travel medicine is still a nascent field. Correspondingly, travel epidemiology and its accompanying methods are also still evolving. Although efforts to collect and distribute data on illnesses in travelers have been successful, challenges and limitations remain. National population-based surveys, or even the addition of questions related to travel, including illness and adverse outcomes experienced during or after travel, might yield promising new data to further define rates and risk in travelers. In 2010, the ISTM research committee published a report summarizing travel medicine research priorities; applied epidemiology, including the work of networks such as GeoSentinel and GTEN, can contribute to providing data to address some of these priorities, including:

What is the benefit of pre-travel counseling (for example, what counseling leads to safer sexual behaviors and fewer road travel accidents, animal bites, drownings, and other noninfectious threats to health)?

How effective is the pre-travel encounter in ensuring that the traveler is up-to-date with locally relevant immunizations (which may or may not be considered travel immunizations)?

What are the risks during medical tourism (such as bloodborne pathogens, nosocomial infections, and procedure complications)?

Do long-term travelers behave differently than short-term travelers?

Do travelers who develop traveler’s diarrhea take their standby antibiotics?

Is the repeated or long-term use of permethrin and other insecticides safe in the ways that travelers use them (on clothing, for example)?

What is the role of travelers in the spread of emerging infections such as chikungunya?